Support explicitly specifying image name

Our initial idea with this module is that it would ignore changes of image because they would be deployed through other means. This is somewhat incompatible with the idea of a move to gitops where we explicitly record the image in the deployment.

The tricksiness here is that the ignore_changes lifecycle argument cannot depend on any expression. We may need to fork the module. Alternatively we could try to be clever and fetch the current image name via a data resource.